Nucleic acids are cellular ingredients necessary for determining genetic make-up and continuity of all living cells. Purine nucleotides are precursors in the biosynthesis of biosynthesis and interconversions of purine nucleotides and their derivatives in microorganisms. Bacterial mutants are used to unravel the essential steps, to obtain genetic modification of the reactions, and to determine the interplay of purine biosynthesis and interconversions with other essential metabolites. Genetic control of the expression of the enzymes is studied by identifying multigenic units of functional control and determining the regulatory elements which control gene expression. This is facilitated by gene fusion of the regulatory regions to genes that code for easily measured enzymes such as beta-galactosidase (lacZ) and galactokinase (galK). Molecular cloning and DNA sequencing techniques are used to facilitate determination of genetic organization and identification of regulatory elements. This is being applied to a general question of how several unlinked genes can be controlled as a unit by common regulatory elements. The special genes and their reactions under currenty study includes: 1) the purE operon, composed of two functional subunits that code for phosphoribosylaminoimidazole carboxylase; 2) the purJHD operon, composed of three genes that are involved in the inosinate cyclohydrolase-transformylase complex (purJH) and phosphoribosylglycineamide synthetase (purD); 3) the inosinate-guanylate reciprocal cycle involving guanylate reductase (guaC) and biosynthesis of guanosine monophosphate (quaAB).